The present invention relates to a gadolinium sputtering target bonded to a titanium backing plate, and to a method of producing such a target.
Gadolinium (Gd) is one of the rare-earth elements, and is present in small amounts in xenotime, monazite and the like. Gadolinium is a gray-colored metal having an atomic number of 64 and an atomic weight of 157.3, and possesses a hexagonal close-packed structure at normal temperature. Moreover, gadolinium has a melting point of 1310° C., a boiling point of 3300° C., density of 7.90 g/cm3, resistivity of 141×10−6 Ωcm, and magnetic susceptibility of 480×10−6 cm3/g. This is a ferromagnetic material at 293 K or less, and has a wide range of application as a raw magnetic material and a raw laser material (refer to The Iwanami's Dictionary of Physics and Chemistry). The linear expansion coefficient of gadolinium is 1.4×10−6/K (20° C.).
In recent years, research and development have been conducted to utilize gadolinium (Gd) as an electronic material (thin film), especially as a metal gate electrode of an FET (field-effect transistor), and gadolinium is a metal that is attracting attention.
As a result of combining gadolinium with a high-permittivity insulating film (high-k insulating film), it is possible to maintain the performance of the transistor at a high level while inhibiting leak currents and generation of heat. When a high-permittivity insulating film (high-k insulating film) is combined with a conventional silicon-based gate electrode, fixation of the carrier and depletion of the gate result from the phenomenon called fermi level pinning that occurs at the interface therebetween, and this causes the malfunction of the transistor or the deterioration in the switching performance due to the increase of the threshold voltage.
Normally, the foregoing electronic material is formed via sputtering. Sputtering is a method in which a DC voltage or a high-frequency voltage is applied between a substrate and a sputtering target while introducing inert gas (mainly Ar gas) into a vacuum, the ionized Ar collides with the sputtering target, and the sputtered target material is deposited onto the substrate.
Upon producing a gadolinium (Gd) sputtering target, the gadolinium ingot prepared by melting and casting is cut into a predetermined size, and this is ground and polished and thereafter bonded with a backing plate to obtain a gadolinium target-backing plate assembly. There are several conventional technologies related to the method of producing a gadolinium sputtering target, and the method of bonding the target with the backing plate.
For example, Patent Document 1 describes that a depressed part is formed in the titanium-alloy backing plate body, and a target is embedded therein and bonded to each other. In the foregoing case, used as the target material are a Cu-based material, an In-based material, a Sn-based material, an Ag-based material, and a Ni-based material. The object of Patent Document 1 is to reduce the recycle cost.
Patent Document 2 discloses technology of using a titanium backing plate for a titanium target, and performing mutual diffusion bonding in order to increase the interface adherence.
Patent Document 3 describes a backing plate for a target of ferromagnetic material, particularly in which a high-permeability material is arranged around the target so that magnetic field lines will not leak from the sputtering face. In Patent Document 3, an example of bonding the rare-earth metal Gd as one example among the numerous ferromagnetic materials to an oxygen-free copper backing plate is illustrated. As a special example, Patent Document 3 also discloses an example of bonding this target to a Ti-alloy backing plate.
Patent Document 4 describes an example of using a titanium target that can be applied to high-power sputtering, and particularly discloses a technology in which high-purity titanium as the target and low-purity titanium as the backing plate are used, and the crystal grain size of the backing plate is made to be smaller than that of the target. The object of Patent Document 4 is to prevent target warpage and detachment.
Moreover, Patent Document 5 describes a method of producing a magnetooptical recording medium, wherein a magnetic film containing multiple materials is deposited by simultaneously discharging and sputtering a plurality of targets, and further describes that a Gd target is used as one of such targets. In the foregoing case, a problem in the stability of the discharge will arise and a compositional variation tends to occur in the deposition. Therefore, an alloy target is used as one of the foregoing targets.
Moreover, Patent Document 6 describes a semiconductor device comprising a Schottky source/drain area formed from a metal in a field-effect transistor, and further describes that the gate electrode is configured from one or more silicides selected from a group containing Er, Yb, Y, Gd, Dy, Ho and La. The object of Patent Document 6 is to lower the threshold voltage of the Schottky source/drain transistor. Although Patent Document 6 describes that Gd is effective as a gate electrode, it does not refer to the issues related to the production of a target.
As described above, it should be understandable that the selection of the target material and the backing plate material is important and numerous devisals have been made in order to improve the deposition rate, stabilize the sputtering process for reducing variation in the deposition, increase the bonding strength between the target material and the backing plate, and prevent the occurrence of warpage and separation of the target material and the backing plate. Nevertheless, gadolinium is a rare-earth element, and gadolinium has attracted attention as a ferromagnetic body so recently that there is no specific disclosure on how to overcome the foregoing problems with gadolinium, nor any suggestion as the solution thereof.
[Patent Document 1] Japanese Laid-Open Patent Publication No. H10-287972
[Patent Document 2] Japanese Laid-Open Patent Publication No. H6-158296
[Patent Document 3] Japanese Laid-Open Patent Publication No. H10-219446
[Patent Document 4] Japanese Laid-Open Patent Publication No. H7-278804
[Patent Document 5] Japanese Laid-Open Patent Publication No. 2000-123432
[Patent Document 6] Japanese Laid-Open Patent Publication No. 2005-209782